The present invention relates to a power transmission unit for wheeled vehicles, and more particularly to a change-speed gearing assembled within a power transmission unit for a motor vehicle of the front-engine, front-wheel drive type.
In recent years, various change-speed gearings have been proposed which are designed to selectively provide multi-speed two-wheel drive for economical driving of the vehicle and multi-speed four-wheel drive for driving of the vehicle on such off-roads as in mountain areas, coast areas and the like. For extrication from an emergency such as a sinking of road-wheels, it is required to develop a change-speed gearing capable of providing low-speed high-torque four-wheel drive. The requirement has hitherto been fulfilled by provision of a power transmission unit which includes a primary change-speed gearing and an auxiliary change-speed gearing mounted on an input member of the former gearing. In the power transmission unit, it is, however, necessary to provide large speed-reduction ratios in the auxiliary change-speed gearing. This results in high torque acting on power transmitting members of the primary change-speed gearing, which requires high strength in the power transmitting members. For the above reasons, the power transmission unit becomes large in size and heavy, and additionally a space for the auxiliary change-speed gearing is required.